There exist at the present time separation problems in food processing, which cannot be adequately solved by using currently available membranes. Many such problems concern the separation of sugars, amino acids and organic acids and bases, from inorganic salts. A further problem concerns the clarification with aggressive solutes, of solutions made from fruit or vegetables; such solutes could damage the membranes in the course of time. Existing membranes have too high rejections of inorganic salts to permit a suitable separation (or if their rejections of inorganic salts is within a desired range, then their rejection of solute which it is desired to concentrate is too low); their application range is limited by their sensitivity to oxidizing agents and solvents; they are not necessarily readily sterilizable by steam or by chemical means; and they require extensive testing before being allowed to come in contact with products intended for human consumption. The present invention provides inter alia membranes which overcome the aforementioned deficiencies of the prior art membranes, and also afford additional advantages. Thus, for example, while many stable food-safe membranes are available with 80+% rejections for molecular weights above 2000, few if any are available which have the desired properties indicated above and at the same time reject molecules of molecular weights below 1500. The present invention provides inter alia membranes in which such molecular weight limits can be advantageously selected as desired.